<3 has been so overused it doesn't even mean what it looks like anymore. People use it when they want to show someone they are happy with them. When you do something for someone and they just give you a <3 it doesn't mean much.

When someone on MSN messenger (or other instant messenger systems like AIM) pretends they are away because they don't want to talk to you, but still want to talk to some people. Basically, it's the less rude form of blocking, but when you block someone they can't see you if you're on.